Water-absorbing resins are widely used in sanitary goods, hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.
Such water-absorbing resins are termed “superabsorbent polymers,” or SAPs, and typically are lightly crosslinked hydrophilic polymers. SAPs are generally discussed in Goldman et al. U.S. Pat. Nos. 5,669,894 and 5,559,335, the disclosures of which are incorporated herein by reference. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirements for an SAP used in a hygienic article, such as a diaper.
As used herein, the term “SAP particles” refers to superabsorbent polymer particles in the dry state, i.e., particles containing from no water up to an amount of water less than the weight of the particles. The term “particles” refers to granules, fibers, flakes, spheres, powders, platelets, and other shapes and forms known to persons skilled in the art of superabsorbent polymers. The terms “SAP gel” and “SAP hydrogel” refer to a superabsorbent polymer in the hydrated state, i.e., particles that have absorbed at least their weight in water, and typically several times their weight in water. The term “surface crosslinking” means that the level of functional crosslinks in the SAP particle in the vicinity of the surface of the particle is generally higher than the level of functional crosslinks in the SAP particle in the interior of the particle. The term “surface-crosslinked SAP particle” refers to an SAP particle having its molecular chains present in the vicinity of the particle surface cross-linked by a compound applied to the surface of the particle.
Initially, the swelling capacity of an SAP particle on contact with liquids, also referred to as free swelling capacity, was the main factor in the design and development of SAP particles. Later, however, it was found that not only is the amount of absorbed liquid important, but the stability of the swollen gel, or gel strength, also is important. The free swelling capacity, on one hand, and the gel strength, on the other hand, represent contrary properties. Accordingly, SAP particles having a particularly high absorbency typically exhibit a poor gel strength, such that the gel deforms under pressure (e.g., the load of a body), and prevents further liquid distribution and absorption.
A balanced relation between absorptivity (gel volume) and gel strength is desired to provide proper liquid absorption, liquid transport, and dryness of a diaper and the skin when using SAP particles in a diaper. In this regard, not only is the ability of SAP particles to retain a liquid under subsequent pressure an important property, but absorption of a liquid against a simultaneously acting pressure, i.e., during liquid absorption also is important. This is the case in practice when a child or adult sits or lies on a sanitary article, or when shear forces are acting on the sanitary article, e.g., leg movements. This absorption property is referred to as absorption under load.
Investigators have researched various methods of improving the amount of liquid absorbed and retained by SAP particles, especially under load, and the rate at which the liquid is absorbed. One preferred method of improving the absorption and retention properties of SAP particles is to surface crosslink the SAP particles.
As understood in the art, surface-crosslinked SAP particles have a higher level of crosslinking in the vicinity of the surface than in the interior. As used herein, “surface” describes the outer-facing boundaries of the particle. For porous SAP particles, exposed internal surface also are included in the definition of surface.
Surface-crosslinked SAP particles under pressure, in general, exhibit higher liquid absorption and retention values than SAP particles having a comparable level of internal crosslinks, but lacking surface crosslinking. Internal crosslinks arise from polymerization of monomers comprising the SAP particles, and are present in the polymer backbone. It has been theorized that surface crosslinking increases the resistance of SAP particles to deformation, thus reducing the degree of contact between surfaces of neighboring SAP particles when the resulting hydrogel is deformed under an external pressure. The degree to which absorption and retention values are enhanced by surface crosslinking is related to the relative amount and distribution of internal and surface crosslinks, and to the particular surface-crosslinking agent and method of surface crosslinking.
The surface crosslinking of SAP particles using crosslinking agents having two or more functional groups capable of reacting with pendant carboxylate or other groups contained on the polymer comprising the SAP particle is disclosed in various patents. For example, U.S. Pat. No. 4,043,952 discloses the use of polyvalent metal compounds as surface-crosslinking compounds. U.S. Pat. No. 4,051,086 discloses the use of glyoxal as a surface crosslinker to improve the absorption rate of SAP particles.
Surface-crosslinking agents include, but are not limited to, diglycidyl ethers, halo epoxy compounds, polyols, polyamines, polyisocyanates, polyfunctional aziridine compounds, and di- or tri-alkylhalides. Regardless of the identity of the surface-cross-linking agent, the agent used for the surface crosslinking has at least two functional groups, and the SAP particles are heated after the surface-crosslinking agent is applied to the surface of the SAP particles.
Prior methods of performing surface cross-linking of SAP particles are disclosed, for example, in U.S. Pat. No. 4,541,871, WO 92/16565, WO 93/05080, U.S. Pat. Nos. 4,824,901; 4,789,861; 4,587,308; 4,734,478; 5,164,459; 4,666,983; 5,385,983; DE 40 20 780, and EP 0 509,708. Surface crosslinking of SAPs is generally discussed in F. L. Buchholz et al., ed., “Modern Superabsorbent Polymer Technology,” Wiley-VCH, New York, N.Y., pages 97-108 (1998).
As disclosed in the art, the SAP particles are either mixed with the surface-crosslinking agent optionally using small amounts of water and/or an organic solvent, or an SAP hydrogel containing 10% to 40%, by weight, water is dispersed in a hydrophilic or hydrophobic solvent and mixed with the surface-crosslinking agent.
One problem encountered in prior methods of surface crosslinking SAP particles is the use of propylene glycol as a cosolvent for the surface-crosslinking agent. Propylene glycol has a relatively high vapor pressure, and is oxidized relatively easily, which adversely affect both the surface-crosslinking method and process equipment, and results in surface-crosslinked SAPs having inconsistent properties. Using propylene glycol as a cosolvent results in the fouling of process equipment attributed to the formation of oxidation and recombination by-products. Propylene glycol volatility also leads to inconsistent and decreased SAP performance.
The present invention is directed to methods of surface-crosslinking SAP particles that overcome the disadvantages associated with prior surface-crosslinking methods which utilize propylene glycol as a cosolvent.